Daniel S. Kahan

The low‐degree shape of Mercury

The shape of Mercury, particularly when combined with its geoid, provides clues to the planets internal structure, thermal evolution, and rotational history. Elevation measurements of the northern hemisphere acquired by the Mercury Laser Altimeter on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft, combined with 378 occultations of radio signals from the spacecraft in the planets southern hemisphere, reveal the low-degree shape of Mercury. Mercurys mean radius is 2439.36 \plusmn 0.02 km, and there is a 0.14 km offset between the planets centers of mass and figure. Mercury is oblate, with a polar radius 1.65 km less than the mean equatorial radius. The difference between the semimajor and semiminor equatorial axes is 1.25 km, with the long axis oriented 15ẹg west of Mercurys dynamically defined principal axis. Mercurys geoid is also oblate and elongated, but it deviates from a sphere by a factor of 10 less than Mercurys shape, implying compensation of elevation variations on a global scale.

A first demonstration of Mars crosslink occultation measurements

A series of three crosslink occultation experiments have been acquired between the Mars Odyssey and Mars Reconnaissance Orbiter spacecraft to probe the Martian atmosphere i\n 2007. While crosslink occultations between Earth orbiting satellites have been used to profile the Earths atmosphere and ionosphere since 1995, this represents the first demonstration of crosslink occultation measurements at another planet. These measurements leverage the proximity link telecommunication payloads on each orbiter, which are nominally used to provide relay communication and navigation services to Mars landers and rovers. Analysis of the observed Doppler shift on each crosslink measurement reveals a clear signature of the Martian atmosphere, primarily the ionosphere. Inversion of the observed Doppler data yields vertical profiles of the Martian refractivity and electron density. The electron density profiles show the presence of two layers with peak densities and peak heights that are consistent with empirical model results. Our study demonstrates the feasibility and future potential of the crosslink radio occultation technique in the exploration of planetary atmospheres.

Direct-to-Earth communications with Mars Science Laboratory during Entry, Descent, and Landing

Mars Science Laboratory (MSL) undergoes extreme heating and acceleration during Entry, Descent, and Landing (EDL) on Mars. Unknown dynamics lead to large Doppler shifts, making communication challenging. During EDL, a special form of Multiple Frequency Shift Keying (MFSK) communication is used for Direct-To-Earth (DTE) communication. The X-band signal is received by the Deep Space Network (DSN) at the Canberra Deep Space Communication complex, then down-converted, digitized, and recorded by open-loop Radio Science Receivers (RSR), and decoded in real-time by the EDL Data Analysis (EDA) System. The EDA uses lock states with configurable Fast Fourier Transforms to acquire and track the signal. RSR configuration and channel allocation is shown. Testing prior to EDL is discussed including software simulations, test bed runs with MSL flight hardware, and the in-flight end-to-end test. EDA configuration parameters and signal dynamics during pre-entry, entry, and parachute deployment are analyzed. RSR and EDA performance during MSL EDL is evaluated, including performance using a single 70-meter DSN antenna and an array of two 34-meter DSN antennas as a back up to the 70-meter antenna.

Demonstration of Mars crosslink occultation measurements for future small spacecraft constellations

Future planetary atmospheric profiling via radio occultations (RO) benefits significantly from increased received signal-to-noise ratio as well as geometrical coverage of links between two or more spacecraft in orbit around a target planet. These can be small spacecraft, possibly dedicated to quality radio-metrics with precision clock reference. Motivated by long-term research investigating the optimum SNR for the science performance, the optimum reference clock stability, the number and combination of wavelengths driving the number of transmitters and antennas, the optimum number and orbital spacecraft configuration, and design modifications to existing radio communication systems that would allow these new science objectives on future missions, three crosslink occultation experiments have been acquired between the Mars Odyssey and Mars Reconnaissance Orbiter spacecraft to probe the Martian atmosphere. While crosslink occultations between Earth orbiting satellites have long been used to profile the Earths atmosphere, this represents the first demonstration of crosslink occultation measurements at another planet. These measurements leverage the proximity link telecommunication payloads on each orbiter, which were designed to provide relay communication and navigation services to Mars landers and rovers. Analysis of the observed Doppler shift on each crosslink measurement reveals a clear signature of the Martian atmosphere, primarily the ionosphere. Inversion of the observed Doppler data yields vertical profiles of the Martian refractivity and electron density. The electron density profiles show the presence of two layers with peak densities and peak heights that are consistent with empirical models.

Sleuthing the MSL EDL performance from an X band carrier perspective

During the Entry, Descent, and Landing (EDL) of NASAs Mars Science Laboratory (MSL), or Curiosity, rover to Gale Crater on Mars on August 6, 2012 UTC, the rover transmitted an X-band signal composed of carrier and tone frequencies and a UHF signal modulated with an 8kbps data stream. During EDL, the spacecrafts orientation is determined by its guidance and mechanical subsystems to ensure that the vehicle land safely at its destination. Although orientation to maximize telecom performance is not possible, antennas are especially designed and mounted to provide the best possible line of sight to Earth and to the Mars orbiters supporting MSLs landing. The tones and data transmitted over these links are selected carefully to reflect the most essential parameters of the vehicles state and the performance of the EDL subsystems for post-EDL reconstruction should no further data transmission from the vehicle be possible. This paper addresses the configuration of the X band receive system used at NASA / JPLs Deep Space Network (DSN) to capture the signal spectrum of MSLs X band carrier and tone signal, examines the MSL vehicle state information obtained from the X band carrier signal only and contrasts the Doppler-derived information against the post-EDL known vehicle state. The paper begins with a description of the MSL EDL sequence of events and discusses the impact of the EDL maneuvers such as guided entry, parachute deploy, and powered descent on the frequency observables expected at the DSN. The range of Doppler dynamics possible is derived from extensive 6 Degrees-Of-Freedom (6 DOF) vehicle state calculations performed by MSLs EDL simulation team. The configuration of the DSNs receive system, using the Radio Science Receivers (RSR) to perform open-loop recording for both for nominal and off-nominal EDL scenarios, is detailed. Expected signal carrier power-to-noise levels during EDL are shown and their impact on signal detection is considered. Particular attention is given to the selection of the appropriate RSR processing bandwidths and to its configuration for real-time signal detection. The X-band carrier frequency obtained through post-processing of the open-loop recorded spectrum is given. Detection of spacecraft status and completion of key vehicle events through their Doppler signature is discussed and illustrated. This Doppler-derived information is compared against the very accurate vehicle data obtained post-EDL via MSLs UHF radio subsystem. The paper concludes with a discussion on the advantages and disadvantages of transmitting the X-band carrier and tone signal in the general context of EDL communications and lessons learned for future missions with EDL sequences are given.

The Challenges and Opportunities for International Cooperative Radio Science; Experience with Mars Express and Venus Express Missions

Radio Science is an opportunistic discipline in the sense that the communication link between a spacecraft and its supporting ground station can be used to probe the intervening media remotely. Radio science has recently expanded to greater, cooperative use of international assets. Mars Express and Venus Express are two such cooperative missions managed by the European Space Agency with broad international science participation supported by NASAs Deep Space Network (DSN) and ESAs tracking network for deep space missions (ESTRAK). This paper provides an overview of the constraints, opportunities, and lessons learned from international cross support of radio science, and it explores techniques for potentially optimizing the resultant data sets.

An innovative direct measurement of the GRAIL absolute timing of Science Data

The Gravity Recovery and Interior Laboratory (GRAIL), a NASA Discovery mission, twin spacecraft were launched on 10 September 2012 and were inserted into lunar orbit on 31 December 2011 and 01 January 2012. The objective of the mission was to measure a high-resolution lunar gravity field using inter-spacecraft range measurements in order to investigate the interior structure of the Moon from crust to core. The first step in the lunar gravity field determination process involved correcting for general relativity, measurement noise, biases and relative & absolute timing. Three independent clocks participated in the process and needed to be correlated after the fact. Measuring the absolute time tags for the GRAIL mission data turned out to be a challenging task primarily because of limited periods when such measurements could be conducted. Unlike the Gravity Recovery and Climate Experiment (GRACE), where absolute timing measurements are available using the GPS system, no absolute timing measurements were available on the far side of the Moon or when there were no DSN coverage periods. During the early cruise phase, it was determined that a direct absolute timing measurement of each spacecraft Lunar Gravity Ranging System (LGRS) clock could be directly observed by using a DSN station to eavesdrop on the Time Transfer System (TTS) S-band inter-satellite ranging signal. By detecting the TTS system directly on earth, the LGRS clock can be correlated directly to Universal Time Coordinated (UTC) because the TTS and LGRS use the same clock to time-tag their measurements. This paper describes the end-to-end preparation process by building and installing a dedicated hardware at Goldstone station DSS-24, selecting favorable lunar orbit geometries, real time signal detection and post processing, and finally how the absolute timing is used in the overall construction of lunar gravity fields.